1. Field of the Disclosure
This disclosure pertains to data communications and particularly to data communications on a wireline such as one employed in an oil or gas well borehole application.
2. Description of the Prior Art
It is common in an oil or gas well borehole application to transmit and receive electrical digital data and control signals between surface electronics and downhole electronics package via a wireline of one or more conductors connecting the two. Such signals are typically used to remotely control the functions of various downhole devices such as sensors for detecting borehole parameters as well as tools and devices for performing functional operations in the borehole such as setting equipment or operating testers, motors, directional drilling equipment or the like, which may be operable in stages and in any event requiring a plurality of differing control signals at different times. Likewise, it is desirable to transmit information indicative of the operation of the downhole devices or parameters detected or measured downhole, to the surface over the same conductor path. It is customary in such downhole operations to utilize a sheathed or armored cable which includes either a single conductor or multiple conductors. A single conductor armored cable typically includes a single insulated conductor as a core, and a protective conductive sheathing surrounds the insulated core. The core and sheathing form an electrical circuit path for transmitting electrical power and data. The standard multi-conductor armored cable is a 7-conductor armored cable used for multiple channel tools. Such so called single conductor wireline cables, or similarly constructed multi-conductor cables, are almost exclusively used to operate downhole electrical devices because of a variety of reasons associated with the space limited and rigorous environment of a borehole. In such oil and gas borehole operations, a borehole depth of many thousands of feet is not uncommon. In communicating between the surface and downhole in a borehole over a wireline cable, control signals and data signals are normally converted to digital signals transmitted by a transmitter at rates up to a maximum of 20 Kbits/second. A receiver on the other end of the cable receives the signals, and a processor decodes the signals for further use.
The transmission and receiver scheme described above operates well when the rate of transmission does not exceed about 20 Kbits/second or the wireline is relatively short. However, the wireline transmission medium does cause a problem when the transmission is over a relatively long length or as the data rate increases. That is, the detection and distinguishing of the two voltage levels associated with the digital signal is impaired by distortions caused by the medium. Distortions become more acute for faster bit rates, where the periods at each of the two voltage levels are very short. For example, the frequency characteristic of a typical single conductor wireline used for downhole application has a loss of about −20 db at 5.6 Khz for a 30,000 foot length. At higher frequencies, the loss is significantly greater.
Often, multi-conductor cables are used when multiple channels to several sensors are used. The most commonly used cable today is a 7-conductor armored logging cable. For comparison purposes, a cable of at least 30,000 feet in length wherein the cable is a 7-conductor cable provided within an armored logging cable having a nominal size of 7/16 inches has a frequency bandwidth of 90 to 270 Khz. Bandwidth is defined as the frequency at which an input signal is attenuated to the point where the signal cannot be effectively recovered by the receiving device. Typically, and for the purposes of this disclosure, the attenuation is −60 db.
Today, while the wells become deeper, the measuring devices have also become more complex. That is, they provide data at a much greater rate. Moreover, the advent of digital computers installed at the well head measuring equipment has enabled the handling of greater volumes of data in a more effective fashion. All of this has occurred simultaneously increasing the requirements on the logging cable. The cables have become more complex i.e., they have added conductors, and the band pass requirements for the conductors have been increased. Still, the cables used today are unable to provide bandwidth in deep wells matching the transmission capabilities of the instrumentation.
There are several factors affecting the bandwidth of a particular cable configuration including resistance (R), capacitance (C), inductance (L) and conductance (or leakage.) Typically gains to be achieved in inductance and conductance are small since these factors are negligible. The most straightforward correction for high resistance of a cable, which is proportional to the diameter cable conductors, is to have larger diameter cables. This correction is opposed by the need to balance cable size with borehole parameters. Parameters such as borehole diameter and fluid pressure lead designers to smaller diameter cables. Capacitance of logging cables has been minimized, thereby increasing bandwidth, by adding conductors or by using a coaxial cable. As discussed earlier, the coaxial cable is used by referencing a signal to the shield (or armor.) Although capacitance is improved, the capacitances of typical coaxial and multi-conductor cables are still around 40 to 60 pF/ft.
Surface communication cables often utilize twisted pairs of conductors to increase bandwidth over single conductor transmission cables. The term twisted pair conductor, as used herein is defined as two electrically-conductive wires, which are electrically insulated from each other and twisted about each other at a given non-zero twist rate. Twisted pair conductors have heretofore been used in downhole applications only with the aide of supporting clamps or structures. One example of a clamped system is U.S. Pat. No. 6,206,133 for “Clamped receiver array using tubing conveyed packer elements”. Another example is U.S. Pat. No. 6,580,751 to Gardner, et al. for “High speed downhole communications network having point to multi-point orthogonal frequency division multiplexing.” The '133 patent describes a geophone array permanently or semi-permanently installed within a well borehole and communicating with a surface computer over twisted wire pairs. Such arrays as described in the '133 and '751 patents are not wireline systems and are unsuitable for self-supporting wireline logging in the drilling phase due to the need quickly insert the wireline data logger into a well borehole, take measurements and then remove the wireline all during a tripping cycle of the drill string.
One problem with implementing twisted pair conductors in a self-supporting wireline is stress induced at each twist crossing point causes conductor deformation or failure at the crossing point when high tensile loads are supplied. Therefore, prior wireline systems are typically designed to a standard wireline cable using single conductors or systems are designed with complicated clamping measures to secure and support the cable during use. An example of a standard wireline cable is described in U.S. Pat. No. 3,259,675 to Bowers for “Method of Manufacturing Armored Cables”. The '675 patent describes a typical 7-conductor wireline cable, which includes a central conductor surrounded by six outer insulated conductors. While the outer conductors are helically wound, they are not twisted pairs as the term is known to those in the art and as the term is used herein.
To address some of the deficiencies described above, the present disclosure provides a load bearing cable having improved bandwidth and lower capacitance per foot for use in wireline applications. This disclosure also provides a multi-conductor load bearing cable used in a single conductor mode with lower capacitance than the typical single conductor cable used today.
Although increasing the bandwidth of a cable is necessary to improve data rate transmission, it should also be appreciated that the efficient use of the bandwidth is also required. As discussed earlier, instruments now have the capability to transmit data at rates far beyond cable capabilities. Methods of encoding data for transmission used in the telecommunication industry include Quadrature Amplitude Modulation (QAM), Carrierless Amplitude and Phase (CAP) modulation, and Discrete Multi-Tones (DMT) modulation. CAP is a modified QAM method, and DMT is the method in digital subscriber line (DSL) applications currently marketed mainly as an enhancement to internet connections. At this time, the well logging community has not taken advantage of the state of the art encoding methods. The primary driver being that the cables in current use cannot provide the bandwidth necessary to utilize these encoding methods efficiently.
To meet the demand for higher data rates, the present disclosure provides a system utilizing telecommunication data encoding methodologies in conjunction with a load bearing data cable having enhanced bandwidth to increase transmission data rate.
This disclosure also provides a method of well logging data transmission having a higher data rate.